Alzheimer's disease (AD) is the prototypic cortical dementia characterized by memory deficit together with dysphasia (language disorder in which there is an impairment of speech and of comprehension of speech), dyspraxia (disability to coordinate and perform certain purposeful movements and gestures in the absence of motor or sensory impairments) and agnosia (ability to recognize objects, persons, sounds, shapes, or smells) attributable to involvement of the cortical association areas. Special symptoms such as spastic paraparesis (weakness affecting the lower extremities) can also be involved [1-4].
Incidence of Alzheimer's disease increases dramatically with the age. AD is at present the most common cause of dementia. It is clinically characterized by a global decline of cognitive function that progresses slowly and leaves end-stage patients bound to bed, incontinent and dependent on custodial care. Death occurs, on average, 9 years after diagnosis [5].
United Nations population projections estimate that the number of people older than 80 years will approach 370 million by the year 2050. Currently, it is estimated that 50% of people older than age 85 years are afflicted with AD. Therefore, more than 100 million people worldwide will suffer from dementia in 50 years. The vast number of people requiring constant care and other services will severely affect medical, monetary and human resources [6].
Memory impairment is the early feature of the disease and involves episodic memory (memory for day-today events). Semantic memory (memory for verbal and visual meaning) is involved later in the disease. By contrast, working memory (short-term memory involving structures and processes used for temporarily storing and manipulating information) and procedural memory (unconscious memory that is long-term memory of skills and procedure) are preserved until late. As the disease progresses, the additional features of language impairment, visual perceptual and spatial deficits, agnosias and apraxias emerge.
The classic picture of Alzheimer's disease is sufficiently characteristic to allow identification in approximately 80% of cases [7]. Nevertheless, clinical heterogeneity does occur and this is not only important for clinical management but provides further implication of specific medication treatments for functionally different forms [8].
The pathological hallmark of AD includes amyloid plaques containing beta-amyloid (Abeta), neurofibrillary tangles (NFT) containing Tau and neuronal and synaptic dysfunction and loss [9-11]. For the last decade, two major hypotheses on the cause of AD have been proposed: the “amyloid cascade hypothesis”, which states that the neurodegenerative process is a series of events triggered by the abnormal processing of the Amyloid Precursor Protein (APP) [12], and the “neuronal cytoskeletal degeneration hypothesis” [13], which proposes that cytoskeletal changes are the triggering events. The most widely accepted theory explaining AD progression remains the amyloid cascade hypothesis [14-16] and AD researchers have mainly focused on determining the mechanisms underlying the toxicity associated with Abeta proteins. Microvascular permeability and remodeling, aberrant angiogenesis and blood-brain barrier (BBB) breakdown have been identified as key events contributing to the APP toxicity in the amyloid cascade [17]. On the contrary, Tau protein has received much less attention from the pharmaceutical industry than amyloid, because of both fundamental and practical concerns. Moreover, synaptic density change is the pathological lesion that better correlates with cognitive impairment than the two others. Studies have revealed that the amyloid pathology appears to progress in a neurotransmitter-specific manner where the cholinergic terminals appear most vulnerable, followed by the glutamatergic terminals and finally by the GABAergic terminals [11]. Glutamate is the most abundant excitatory neurotransmitter in the mammalian nervous system. Under pathological conditions, its abnormal accumulation in the synaptic cleft leads to glutamate receptor overactivation [18]. Abnormal accumulation of glutamate in synaptic clefts leads to the overactivation of glutamate receptors that results in pathological processes and finally in neuronal cell death. This process, named excitotoxicity, is commonly observed in neuronal tissues during acute and chronic neurological disorders.
It is becoming evident that excitotoxicity is involved in the pathogenesis of multiple disorders of various etiology such as: spinal cord injury, stroke, traumatic brain injury, hearing loss, alcoholism and alcohol withdrawal, alcoholic neuropathy, or neuropathic pain as well as neurodegenerative diseases such as frontotemporal dementia, multiple sclerosis, Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and Huntington's disease [19-21]. The development of efficient treatment for these diseases remains a major public health issue due to their incidence as well as lack of curative treatments.
Two kinds of medication are used for improving or slowing down symptoms of AD which lay on some acetylcholinesterase modulators and a blocker of NMDA glutamate receptors, memantine [22,23].
NMDAR antagonists that target various sites of this receptor have been tested to counteract excitotoxicity. Uncompetitive NMDAR antagonists target the ion channel pore, thus reducing calcium entry into postsynaptic neurons. Some of them reached the approval status. As an example, memantine is currently approved in moderate to severe Alzheimer's disease. It is clinically tested in other indications that include a component of excitotoxicity such as alcohol dependence (phase II), amyotrophic lateral sclerosis (phase III), dementia associated with Parkinson's disease (phase II), epilepsy, Huntington's disease (phase IV), multiple sclerosis (phase IV), Parkinson's disease (phase IV) and traumatic brain injury (phase IV). This molecule is however of limited benefit to most Alzheimer's disease patients, because it has only modest symptomatic effects. Another approach in limiting excitotoxicity consists of inhibiting the presynaptic release of glutamate. Riluzole, currently approved in amyotrophic lateral sclerosis, showed encouraging results in ischemia and traumatic brain injury models [24-26]. It is at present tested in phase II trials in early multiple sclerosis, Parkinson's disease (does not show any better results than placebo) as well as spinal cord injury. In 1995, the drug reached orphan drug status for the treatment of amyotrophic lateral sclerosis and in 1996 for the treatment of Huntington's disease. The use of NMDA receptor antagonists such as memantine, felbamate, acamprosate and MRZ 2/579 for treating depression has also been suggested in US2010076075.
WO2009133128, WO2009133141, WO2009133142 and WO2011054759 disclose drug combinations for use in the treatment of AD.
Despite active research in this area, there is still a need for alternative or improved efficient therapies for neurological disorders and, in particular, neurological disorders which are related to glutamate and/or amyloid beta toxicity. The present invention provides new treatments for such neurological diseases of the central nervous system (CNS) and the peripheral nervous system (PNS).